Various control technologies for controlling electric power steering have been developed (c.f. JP 2016-580 A and JP 2016-22927 A). JP 2016-580 A discloses technologies for reducing a calculation load for calculating an assist torque which assists steering of a driver. JP 2016-22927 A discloses control technologies which do not only output appropriate assist torque but also have a function of reducing vibration which is transmitted to a steering wheel.
FIG. 9 shows a graph used for performance evaluation of a steering mechanism mounted on a vehicle. The performance evaluation of the steering mechanism is described with reference to FIG. 9.
The horizontal axis of the graph of FIG. 9 represents a steering angle (i.e. a rotation angle of a steering wheel). The steering angle of “0°” means a rotation position of the steering wheel when the vehicle runs straightforwardly. With regard to the graph of FIG. 9, a driver rotates the steering wheel by “120°”. The steering wheel is returned to the steering angle of “0°” after being held at the steering angle of “120°”.
The vertical axis of the graph of FIG. 9 represents a required torque which is required for the driver to apply to the steering wheel during the aforementioned steering wheel operation. The graph of FIG. 9 shows two curves FWC, RTC. The curve FWC indicates a change in required torque under a change in steering angle from “0°” to “120°”. The curve RTC indicates a change in required torque under a change in steering angle from “120°” to “0°”.
The difference in required torque between the curves FWC, RTC is referred to as “hysteresis”. The hysteresis is used as an index for evaluating performance of the steering mechanism. When the hysteresis is excessively large at the steering angle of “120°”, there is worse responsiveness of the steering mechanism.
The aforementioned control technologies do not address a reduction in hysteresis. The reduction in hysteresis under the aforementioned control technologies requires changes of various control parameters.
It is an object of the present invention to provide control technologies for easily reducing the hysteresis.